Wettability of a Polymethylmethacrylate Surface by Extended Anionic Surfactants: Effect of Branched Chains

The adsorption behaviors of extended anionic surfactants linear sodium dodecyl(polyoxyisopropene)₄ sulfate (L-C₁₂PO₄S), branched sodium dodecyl(polyoxyisopropene)₄ sulfate (G-C₁₂PO₄S), and branched sodium hexadecyl(polyoxyisopropene)₄ sulfate (G-C₁₆PO₄S) on polymethylmethacrylate (PMMA) surface have been studied. The effect of branched alkyl chain on the wettability of the PMMA surface has been explored. To obtain the adsorption parameters such as the adhesional tension and PMMA-solution interfacial tension, the surface tension and contact angles were measured. The experimental results demonstrate that the special properties of polyoxypropene (PO) groups improve the polar interactions and allow the extended surfactant molecules to gradually adsorb on the PMMA surface by polar heads. Therefore, the hydrophobic chains will point to water and the solid surface is modified to be hydrophobic. Besides, the adsorption amounts of the three extended anionic surfactants at the PMMA-liquid interface are all about 1/3 of those at the air-liquid interface before the critical micelle concentration (CMC). However, these extended surfactants will transform their original adsorption behavior after CMC. The surfactant molecules will interact with the PMMA surface with the hydrophilic heads towards water and are prone to form aggregations at the PMMA-liquid interface. Therefore, the PMMA surface will be more hydrophilic after CMC. In the three surfactants, the branched G-C₁₆PO₄S with two long alkyl chains exhibits the strongest hydrophobic modification capacity. The linear L-C₁₂PO₄S is more likely to densely adsorb at the PMMA-liquid interface than the branched surfactants, thus L-C₁₂PO₄S possesses the strongest hydrophilic modification ability and shows smaller contact angles on PMMA surface at high concentrations.